Continued adoption of cellular systems for data transfer as well as voice communications along with introduction of new mobile communications devices such as Tablet devices make cellular coverage in urban environments a priority. In particular, improving cellular coverage indoors is important to provide a seamless user experience in the mobile communication arena. Distributed antenna systems (DAS) are being installed in office buildings and public areas and are used to provide stronger RF signals to improve the communication link for cellular and data services.
Initial DAS antenna systems were only required to operate over a few frequency bands, making the antenna design process easier. As the communications industry has moved from 2G to 3G cellular systems, and with the advent of 4G communication systems such as Long Term Evolution (LTE), additional frequency bands are required from a DAS antenna system which increases the difficulty in terms of antenna design. With the adoption of 4G LTE cellular systems the need for a two antenna assembly to provide MIMO (Multiple Input Multiple Output) capability is required for in-building DAS systems. This requirement for a two antenna pair at multiple locations for in-building applications puts more importance on antenna assembly size reduction to minimize visual impact of these antennas when a full system is installed.
As communication systems such as DAS transition to MIMO capability to assist in servicing a growing demand for higher data rates for in-building mobile communication users, and as more users access high data rate features such as file sharing and video downloads the signal to noise characteristics and RF signal levels of the cellular signals indoors become more important parameters. To maintain low noise floors in communication systems a parameter that is important to address in the antenna design is Passive Intermodulation (PIM). PIM products are generated when two RF signals at different frequencies are injected into an antenna port; the antenna, though being a passive device, can generate spurious responses due to “natural diode” junctions in the antenna. These natural diode junctions can be formed at the junction of two metal surfaces where the metals are dissimilar. Corrosion and oxidation at these junctions can also cause spurious frequency components due to mixing of the two RF signals. Proper antenna design and material selection is important to meet stringent, low PIM requirements. As PIM components increase, these spurious frequency components add to the noise level, which in turn results in reduced signal to noise ratio of the communication system. This will result in reduced data rates for users.
The desire for a small form factor MIMO antenna system that can cover wide frequency ranges and possess wide beamwidth characteristics across these wide frequency ranges brings difficult design challenges in terms of maintaining high port to port isolation for the antenna pair as well as maintaining low envelope correlation coefficient (ECC). Maintaining the isolation and ECC requirements are key to providing the antenna characteristics needed on the base station or node side of the communication link to achieve the increased data rates a MIMO communication system can delivered compared to SISO (Single Input Single Output) systems. Port to port isolation in particular can be difficult to achieve when wide frequency bandwidths are required and the inter-element spacing is small. With isolation typically being dependent on antenna element separations as a function of a wavelength, maintaining acceptable isolation at the lower frequency bands can be the challenge as well as degraded isolation at narrow band regions at the higher frequencies when wide frequency bandwidths are attempted in an antenna system design.